1. Field of the Invention
The disclosed technology relates to electrochemical ethylene sensors, to the use of electrochemical sensors for ethylene monitoring, and to methods for ethylene monitoring.
2. Description of the Related Technology
Ethylene monitoring can be very useful in fruit quality control. Ethylene is a plant hormone that is excreted by ripe fruit. It can induce undesired ripening (leading to a reduced lifetime) in other fruits stored in the vicinity. Conversely, ethylene gas is sometimes added into warehouses to artificially induce ripening. Ethylene monitoring is therefore very useful for monitoring fruit freshness.
Ethylene can be oxidized and is therefore amenable for electrochemical detection.
Electrochemical gas detection is based on oxidation or reduction of a target gas at an appropriately biased electrode. An electrochemical gas sensor typically comprises three electrodes: a working electrode, a reference electrode and a counter electrode, the electrodes being in contact with an electrolyte. Before electrochemically reacting at the working electrode surface, the target gas dissolves in the electrolyte. The electrochemical reaction results in an electric current, that is a measure for the amount of gas oxidized or reduced at the working electrode. The power consumption of this type of sensors is intrinsically small and therefore well suited for emerging wireless, ultra-low power autonomous transducer systems.
However, this type of sensors has several disadvantages. For example, for many gases, such as carbon monoxide, hydrogen sulphide, nitric oxide and ethylene, electrochemical detection requires a reservoir filled with a high-molarity acidic electrolyte such as a sulfuric acid solution. This solution is irritating at the concentrations used, thus imposing strict requirements on the reservoir package. Furthermore, the solubility of these gases in water is limited and therefore a large working electrode is required to achieve the desired detection range. The large working electrode in combination with a dangerous concentration of sulfuric acid in the reservoir leads to bulky sensors. In addition, a liquid used as an electrolyte evaporates, which leads to drift and eventually sensor failure.
The first step of the mechanism of electrochemical ethylene sensing consists of ethylene adsorption at the working electrode surface followed by several electron-transfer events. The oxidation of the working electrode at high applied potentials plays a crucial role in the functioning of electrochemical ethylene sensors because it hinders ethylene oxidation. For example, gold is able to oxidize ethylene at room temperature only in an acidic electrolyte, because only in an acid environment a potential window exists in which ethylene oxidation can occur before the onset of gold oxidation.
In food quality monitoring however, the use of a strong acidic electrolyte is undesirable or it would impose strict requirements on the sensor package. There is a need for simple, accurate, cheap and stable ethylene sensors that are small and have low power consumption.
Ethylene can for example be detected and monitored by means of sensors comprising a semiconducting metal oxide layer, wherein the detection is based on measuring the metal oxide resistivity. However, this type of sensors requires an elevated operating temperature, resulting in relatively high power consumption. The response time and the recovery time are relatively large, e.g. in the order of a few minutes.